he was touching her over the pants but she didn't touch him at all back.
She never really had a chance in her teens to go crazy
her head was all fuzzy and it was a random and exciting unexpected event
She also said she wants to say she'd never do anything like that in the future, but is not 100% sure
Read that back and tell me that you've got self respect? That your gf respects you or your relationship?
"It just happened. So random." Did flirting all night, sneaking off to a private room with 1 guy who she's sexually interested in just happen as well? How about sitting on a bed next to him and getting closer and closer? How about the decision to keep going after 10 seconds, a minute, 3 minutes of snogging? How about deciding to keep going when he started groping her?
It all just happened to her didn't it. And the kicker is that she's literally told you that she doesn't respect you or your relationship enough to not do it again!
Get rid of her. You're worth more than this.